In certain video disc systems, video information is recorded by means of geometric variations in the bottom of a smooth spiral groove on the surface of a disc record. The disc record surface includes a coating of conductive material which is preferably covered with a thin deposit of dielectric material. A signal pickup, supported by a pickup arm, engages the spiral groove and includes a conductive electrode which establishes a capacitance with the conductive coating and the dielectric deposit of the disc record. When the disc record is rotated, the electrode/disc capacitance varies in response to the geometric variations in the bottom of the spiral groove passing beneath. The capacitance variations are converted to electrical variations by a suitable signal processing circuitry coupled to the signal pickup electrode. The output signal of the signal processing circuitry may be coupled to a conventional television receiver for reproduction. The other end of the pickup arm is releasably secured by a coupler to a support member of a supporting structure of the playback system. A system of the aforementioned type is described in detail in the U.S. Pat. No. 3,842,194, issued on Oct. 15, 1974, to J. K. Clemens.
Video disc systems of the aforementioned type generally utilize disc records having groove densities in the order of four to eight thousand groove convolutions per inch. A typical video disc record of this type may have a groove convolution spacing in the order of 3.5 microns. The fragile walls of relatively narrow grooves of the disc record cannot be dependably relied upon to pull the weight of the pickup arm assembly, around the pickup arm pivot support, across the entire recorded surface of the disc record. Also, in video disc systems utilizing the variable capacitor concept, it is desirable for accurate reproduction of the prerecorded signals that the signal pickup electrode maintain a substantially constant attitude in the spiral groove. Stated differently, the position and the angular orientation of the signal pickup electrode in relation to the information track in the spiral groove must be held relatively constant. Therefore, the supporting structure includes a radial feed drive mechanism for traversing the supported end of the pickup arm in proper time relationship with the radial motion of the signal pickup tip engaged in the spiral groove so as to continuously maintain the longitudinal axis of the pickup arm substantially tangential to the spiral groove at the point of engagement. Reference may be made to the copending U.S. Application of F. R. Stave, Ser. No. 351,600, filed Apr. 16, 1973, entitled "VIDEO DISC PLAYBACK APPARATUS", and now U.S. Pat. No. 3,870,835, for an illustration of a suitable radial feed drive mechanism for providing the indicated radial motion.
Further, in the above-mentioned type video disc systems, it has been recognized that the relative motion between the disc record and the signal pickup must be maintained at a predetermined speed, and within specified tolerance limits (e.g., 450 rpm, .+-.0.01 percent), in order to obtain high fidelity of reproduction of the prerecorded signals. The predetermined speed and the specified tolerance limits are also necessary to assure that the horizontal and vertical synchronizing information is stable and within the lockup range of the deflection circuits of the television receiver. Moreover, when the prerecorded information is a color television signal with chrominance information recorded as a modulated carrier signal, the recovered signal must be stable and within the lockup range of the color processing circuits of the playback system in order to reduce color phase distortion.
The disc record/signal pickup relative speed may be maintained at the predetermined speed, and within the specified tolerance limits by rendering the pickup arm support member subject to cyclical, translatory motion along the longitudinal axis of the pickup arm in a manner that opposes deviations of the instantaneous relative speed from the predetermined speed. Illustratively, the means for imparting translatory motion to the support member may be of the type disclosed in the U.S. Pat. No. 3,711,641, issued to R. C. Palmer on Jan. 16, 1973, entitled "VELOCITY ADJUSTING SYSTEM".
Additionally, in an advantageous pickup arrangement for video disc systems of the aforementioned type, the pickup arm is made of conductive material and is enclosed in a conductive cage. The conductive pickup arm and the surrounding conductive cage serve respectively as an inner and outer conductor of an electrical transmission line. The transmission line is capacity end loaded at the pickup arm supported end by a series combination of capacitances which include an air dielectric capacitor and a voltage variable capacitor (for tuning purposes). The transmission line is also capacitor end loaded at the signal pickup end by a combination of capacitances which include the signal pickup electrode/disc record conductive coating variable capacitance. The transmission line and associated capacitances form a tuned circuit with a resonant frequency subject to variations as the signal pickup electrode/disc conductive coating capacitance varies. The tuned circuit is excited with UHF oscillations from a fixed frequency oscillator of the signal processing circuitry operating at a frequency (e.g., 915 MHz) within an ISM-allocated band. As the resonant frequency of the tuned circuit varies, the resultant UHF oscillation amplitude variations are detected by the signal processing circuitry detector to recover the prerecorded information. Reference may be made to the copending U.S. application of D. J. Carlson, et al., Ser. No. 451,103, filed Mar. 14, 1974, entitled "PICKUP APPARATUS FOR VIDEO DISC PLAYERS", and now U.S. Pat. No. 3,872,240, for a more detailed description of the aforesaid transmission line arrangement and the associated circuitry.
In playback systems of the aforementioned type, the following considerations may be encountered. First, groove-engaging and low mass requirements of such a typical signal pickup apparatus utilized in the above-mentioned systems result in a pickup arm unit which is extremely fragile (e.g., signal pickup width 2 microns, signal pickup depth 5 microns, signal pickup electrode width 2 microns, and signal pickup electrode depth 0.2 microns). Therefore, it is desirable to protect the pickup arm unit.
Second, the signal pickup must maintain the specified attitude in the spiral groove during playback in order to preclude undesirable variations (i.e., not representative of prerecorded signals in the information track) in signal pickup electrode/disc record conductive coating capacitance. Therefore, it is desirable to facilitate replacement of the miniature signal pickup electrode (e.g., electrode width 2 microns, and electrode depth 0.2 microns) without need for any manual, electrode attitude adjustments.
Third, the conductive pickup arm, electrically connected to the signal pickup electrode, and the signal pickup may desirably be treated as a replaceable unit of a size convenient to handle, whereby when the signal pickup wear calls for a replacement, the player user may readily remove and replace the pickup arm unit without requirements for mechanical skill and manual dexterity. A replaceable unit arrangement may be desirable for several reasons. One, the replacement of the pickup arm/signal pickup unit would require no electrical wiring disconnections and connections between the signal pickup electrode and the conductive pickup arm. Two, achievement of a proper attitude for the signal pickup in its playing position may readily be established, and not left to dependence on the skill and knowledge of the user during replacement. Three, the replacement of the fragile and miniature signal pickup (e.g., the signal pickup width 2 microns, the signal pickup depth 5 microns, and the signal pickup electrode depth 0.2 microns) would be rendered more practicable.
Fourth, it would be also desirable to facilitate replacement of the pickup arm unit without need for any manual, complicated disconnection and connection between the pickup arm unit and the translatory motion imparting means, whereby unskilled consumer replacement of a replaceable unit becomes more feasible.
In a copending U.S. application of M. A. Leedom, Ser. No. 522,815, filed concurrently, and entitled "PICKUP ARM CARTRIDGE", and assigned to the present assignee, a replaceable cartridge arrangement is described. In the Leedom arrangement, the pickup arm coupler is suspended in the cartridge casing by an elastic diaphragm. The elastic diaphragm maintains angular orientation of the signal pickup while permitting arcuate and translatory motion of the pickup arm unit throughout playback. Further, a leaf spring is utilized for securing the pickup arm unit in the casing during storage and handling of the cartridge. The leaf spring also serves to lower the pickup arm so that the signal pickup may ride in the spiral groove.
In another copending application of B. K. Taylor, et al., Ser. No. 522,822, filed concurrently, entitled "DETACHABLE PICKUP ARM MAGNETIC COUPLING", and also assigned to the present assignee, an arrangement for orienting and detachably coupling the pickup arm unit coupler with the translatory motion imparting means support member is described.
The present invention is concerned with a modification of the pickup arm cartridge and the detachable pickup arm magnetic coupling of the general form described, in the aforesaid, respectively, Leedom and Taylor, et al., applications to provide a pickup arm cartridge apparatus of a modified form. The present invention provides novel means which suspend the pickup arm unit within the casing during storage and handling of the cartridge while not interfering with the pickup arm unit arcuate and translatory motion during playback. The present invention further provides novel means for disabling the clamping means (which securely hold the pickup arm unit in the casing) during assembly of the cartridge in the player. Separate means are provided in the associated player for carefully lowering the pickup arm for engagement of the signal pickup with the spiral groove.